8. SOME THEORIES AND PROBLEMS WITH EACH

Space does not allow even a superficial discussion of many individual
theories interpreting the observational evidence summarized above. We
summarize a few.

1. Bare silicate/graphite grains:DL84,
in a very careful discussion of
the optical constants of both graphite and silicates, greatly extended
in wavelength an older theory of Mathis, Rumpl, and Nordsieck
(114;
often referred to as ``MRN''). The features of the theory are that
(a) individual grains are bare and homogeneous, composed of either
silicate or graphite; (b) the size distribution is a power-law, where
n(a), the number of grains of radius a, is proportional to
a-3.5; and (c)
the size distribution is truncated at the upper end at 0.25
µm, with the
lower end of sizes extended downward to a few angstroms to
fit the IRAS data
(41,
133),
and very likely all the way to PAHs in order to produce the UIBs.

DL84 subjected
their theory to a much more exacting comparison
with observations than did most other authors. The fit to the
extinction law for diffuse dust, over the entire observed
wavelength range from 0.1 µm to 1000 µm, is
impressively good. On the other had,
DL84
requires that large graphite particles be
produced from the amorphous carbon late-type stars inject into
the ISM. It is very difficult to understand how the necessary
annealing can take place under interstellar conditions.
Furthermore, the materials in the two types of grains (silicates
and graphite) must be kept separate, in spite of the many cycles
of coagulation and rearrangement of the size distributions which
take place as the grains cycle into and out of clouds.

2. Core/mantle grains: Greenberg and his collaborators (see
references in ref.
62)
believe that the bulk of interstellar grains have
refractory silicate cores covered with an organic refractory mantle.
This mantle is produced from the processing by both UV photons and
cosmic rays deep inside molecular clouds, after the icy mantles observed
in such clouds are deposited upon the grain surfaces. Laboratory
studies show that molecules in such mantles can be partially converted
into free radicals that are chemically active enough to react violently
when warmed, producing complex molecules
(31).
Such runaway reactions could be triggered on inner-cloud grains by
cosmic rays. After such an event, a organic polymeric substance known
as ``organic refractory'' material, stable at room temperature, remains.
In this theory, organic refractory mantles on silicate cores produce the
optical/NIR extinction, while the bump is produced by small graphite
particles, PAHs produce the UIBs, and the shortest-wavelength extinction
is from small particles, probably silicates. Chlewicki and Laureijs
(23)
suggest that an additional component of iron will produce most
of the 60-µm emission observed by IRAS.

These ideas have a great deal of appeal as regards events deep within
clouds. The Greenberg scenario explains why interstellar molecules are
found in the gas phase within dense clouds, where they should freeze
onto grains in a short time - the runaway reactions drive off the
molecules, and gas-phase chemistry takes place before refreezing onto
grain surfaces. An alternative explanation of gas-phase molecules
inside dense clouds is a very rapid circulation of grains between the
surface and the center of the cloud (e.g.,
21).

There are four problems as regards extending these ideas into diffuse
dust: (a) Grains are larger in outer-cloud dust because of coagulation,
not accretion of mantles, as shown by the reduced extinction per H atom
in some cases. (b) Organic refractory mantles, which are less
refractory than silicates and solid carbon, would be more readily
destroyed by shocks in the diffuse ISM. The destruction rate of
materials depends sensitively upon the binding energy
(43,
44).
(c) Solar system dust particles suggest
that the silicate and carbon materials coagulate into large structures
before icy mantles envelop them. (d) The 3.4-µm C-H
absorption band,
seen in organic refractory material in the laboratory along with the
3.08-µm ice band, is locally weak or absent. The object with the
strongest 3.4-µm band, IRS 7 near the galactic center
(17),
is not typical of local dust. The 3.08- and 3.4-µm bands
are not yet seen towards the local star VI Cyg 12
(61)
[for which A(V) 10 mag],
limiting these bands to less than 0.3 the strength, per
A(V), of those for IRS 7. The 3.4-µm band is seen in
Lynga 8/IRS 3
[154;
A(V) = 17], about 0.4 times as strong as for IRS 7. The
3.4-µm band is always accompanied by a stronger
3.08-µm ice feature, and there are some lines of sight
(64)
with no ice band for A(V) < 20 mag.

It is possible
(156)
that the organic refractory mantles are
so heavily processed that they lose almost all of their N and O,
becoming essentially amorphous carbon. This material would be difficult
to distinguish from amorphous C injected directly into the ISM from
carbon stars. In this case, the Greenberg theory is very similar to
composite-grain theories (see below).

3. Silicate cores with amorphous carbon mantles: Duley et al.
(47)
suggest that grains are silicates with mantles of
hydrogenated amorphous carbon (HAC). One population is very small and
produces the bump by (OH)- ion absorption in the presence of
Si atoms. (All other theories produce the bump from well-ordered carbon). The
UIBs are caused by absorption of UV photons by ``islands'' of HAC on the
silicate core surfaces, so thermally isolated that they can radiate like
free particles [for about 1 second!
(133)].
The rapid increase of extinction
with wavenumber for -1 > 6 µm-1 is produced
by diamond-like bonding in the ``amorphous'' C.

This theory makes several predictions that can be tested. It explains
the differences between diffuse dust and outer-cloud dust rather
naturally, as arising from different depletions of carbon onto the
silicate cores. However, it requires a very large fraction of the Si
atoms to have OH- ions nearby, near the surfaces of small
grains, even if the bump transition in OH- has an oscillator
strength of unity. The
thermal isolation of the ``islands'' of HAC is difficult to achieve.

4. Composite grains: Mathis and Whiffen
(113)
and Tielens
(156)
suggest that interstellar grains consist of an assembly of
small particles of carbon and silicates, jumbled together loosely.
These grains are the natural result of coagulation and disruption of
grains as they cycle into clouds. The particles inside the porous
structure are protected from shocks and might well be covered with
highly processed organic material. The bump is provided by small
graphitic particles; PAHs can produce the UIBs. The rise in extinction
for < 0.16 µm is
provided by the diamond bonding in ``amorphous'' C. The
composite grains are mostly open, in analogy with interplanetary dust
particles. However, too much porosity provides too large an opacity in
the FIR, making the grains too cold because they radiate efficiently.
It is difficult to calculate the extinction of composite grains, so the
calculated fit should be taken as provisional.

5. Fractal grains: Wright
(182)
suggested that interstellar grains
are the product of coagulation into very large fractal structures
resembling twisted branches
(65).
If one defines the fractal dimension, , by MR, then depends upon the sequence
of coagulation (and the probability of the fractal grains' breaking up,
neglected in the calculations). In general < 3, and in some cases <
2. One of the major features of fractal grains is an FIR absorption per
unit mass larger by an order of magnitude or more over solid grains.

Fractal grains can explain very large radar backscattering in comets
(183)
without large masses of dust. They also explain the
very shallow (-1) dependence of the opacity submillimeter opacity
observed in some very dense nebulae
(Section 3.2.3).
However, the FIR
opacity of fractal grains is so large that the grains would be too cold
to explain the observed FIR spectrum of galactic dust (T 20 K).

6. Biological grains: Hoyle, N.C. Wickramasinghe, and others
(76,
161,
and references therein) have
suggested that the grains producing visual extinction have a biological
origin, with the bump provided by graphite. The extinction and
polarization laws are fitted reasonably well. However, there are two problems
with the model: (a) There is not enough cosmic phosphorous to
accommodate the amount found in organisms
(45,
164;
but see
74).
The cosmic abundance of
P is low, and most of it is in the gas phase for low-density lines of
sight, so this criticism seems valid. (b) Organisms, even when dried,
show strong O-H and C-H stretch absorptions
(75),
which are not seen except deep within molecular clouds.